1. Field of the Invention
The present invention relates to an optical pickup apparatus compatible with a digital video disk (DVD) and a recordable compact disk (CD-R), and more particularly, to an optical pickup apparatus which can compatibly record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively, using a holographic lens.
2. Description of the Related Art
An optical pickup apparatus records and reads the information such as video, audio or data at a high density, and various types of recording media are a disk, a card and a tape. Among them, the disk type is primarily used. Recently, in the field of the optical disk apparatus, a laser disk (LD), a compact disk (CD) and a digital video disk (DVD) have been developed. Such an optical disk includes a plastic or glass medium having a certain thickness along an axial direction to which light is incident, and a signal recording surface on which information is recorded and located on the plastic or glass medium.
So far, a high-density optical disk system enlarges a numerical aperture of an objective lens to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. Accordingly, the high-density optical disk system can record or read signals on or from a digital video disk, and can also read signals from a CD. However, to be compatible with a recent type of a CD, that is, a recordable CD (CD-R), light having a wavelength of 780 nm should be used, due to the recording characteristic of the CD-R recording medium. As a result, using the light beam wavelengths of 780 nm and 635 (or 650) nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG. 1.
FIG. 1 shows an optical pickup using two laser light diodes as light sources for a DVD and a CD-R and a single objective lens. The FIG. 1 optical pickup uses laser light having a wavelength of 635 nm when reproducing a DVD, and uses laser light having a wavelength of 780 nm when recording and reproducing a CD-R.
Light having the 635 nm wavelength emitted from a first laser light source 11 is incident to a first collimating lens 12, in which the light is shown in a solid line. The first collimating lens 12 collimates the incident light beam to be in a parallel light beam. The light beam passing through the first collimating lens 12 is reflected by a beam splitter 13 and then goes to an interference filter prism 14.
Light having the 780 nm wavelength emitted from a second laser light source 21 passes through a second collimating lens 22, a beam splitter 23 and a converging lens 24, and then goes to the interference filter prism 14, in which the light is shown in a dotted line. Here, the light beam of the 780 nm wavelength is converged by the interference filter prism 14. An optical system having such a structure is called a "finite optical system." The interference filter prism 14 totally transmits the light beam of the 635 nm wavelength reflected from the beam splitter 13, and totally reflects the light beam of the 780 nm wavelength converged by the converging lens 24. As a result, the light beam outgoing from the first laser light source 11 is incident to a quarter-wave plate 15 in the form of a parallel beam by the collimating lens 12, while the light beam from the second laser light source 21 is incident to the quarter-wave plate 15 in the form of a divergent beam by the converging lens 24 and the interference filter prism 14. The light transmitted through the quarter-wave plate 15 passes through a variable aperture 16 having a thin film structure and then is incident to an objective lens 17.
The light beam of the 635 nm wavelength emitted from the first laser light source 11 is focussed by the objective lens 17 on an information recording surface in the DVD 18 having a thickness of 0.6 mm. Therefore, the light reflected from the information recording surface of the DVD 18 contains information recorded on the information recording surface. The reflected light is transmitted by the beam splitter 13, and is then incident to a photodetector 19 for detecting optical information.
If the finite optical system described above is not used, when the light beam of the 780 nm wavelength emitted from the second laser light source 21 is focused on an information recording surface in the CD-R 25 having a thickness of 1.2 mm using the above-described objective lens 17, spherical aberration is generated due to a difference in thickness between the DVD 18 and the CD-R 25. The spherical aberration is due to the fact that the distance between the information recording surface of the CD-R 25 and the objective lens 17 is farther than that between the information recording surface of the DVD 18 and the objective lens 17, along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including the converging lens 24 is required. By using the variable aperture 16 to be described later with reference to FIG. 2, the light beam of the 780 nm wavelength forms an optimized beam spot on the information recording surface of the CD-R 25. The light beam of the 780 nm wavelength reflected from the CD-R 25 is reflected by the beam splitter 23, and then detected in a photodetector 26.
The thin-film type variable aperture 16 of FIG. 1, as shown in FIG. 2, has a structure which can selectively transmit the light beams incident to the regions whose numerical aperture (NA) is less than or equal to 0.6, which coincides with the diameter of the objective lens 17. That is, the variable aperture 16 is partitioned into two regions based on the numerical aperture (NA) of 0.45 with respect to an optical axis. Among the two regions, a first region 1 transmits both light beams of 635 nm wavelength and 780 nm wavelength. A second region 2 totally transmits the light beam of the 635 nm wavelength and totally reflects the light beam of the 780 nm wavelength. The region 1 is a region having a numerical aperture less than or equal to 0.45, and the region 2 is an outer region relative to the region 1 in which a dielectric thin film is coated. The region 1 is comprised of a quartz (SiO.sub.2) thin film to remove any optical aberration generated by the dielectric thin film coated region 2.
By using the variable aperture 16, the 780 nm wavelength light transmitted through the region 1 having the 0.45 NA or below forms a beam spot appropriate to the CD-R 25 on the information recording surface thereof. Thus, the FIG. 1 optical pickup uses an optimum beam spot when a disk mode is changed from the DVD 18 to the CD-R 25. Accordingly, the FIG. 1 optical pickup is compatible for use with the CD-R.
However, the optical pickup shown in FIG. 1 and as described above should form a "finite optical system" with respect to the 780 nm wavelength light in order to remove any spherical aberration generated when changing a DVD compatibly with a CD-R. Also, due to the optical thin film, that is, the dielectric thin film, which is formed in the region 2 of the variable aperture 16 having the NA of 0.45 or above, an optical path difference between the light transmitted through the region 1 having the NA of 0.45 or below and that transmitted through the region 2 having the NA of 0.45 or above, is generated. To eradicate this difference, it is necessary to form an optical thin film in the region 1. Due to this reason, a quartz coating (SiO.sub.2) is formed in the region 1 and a multi-layer thin film is formed in the region 2. However, such a fabricating process does not only become complicated but also adjustment of the thickness of the thin film should be performed precisely in units of ".mu.m". Thus, it has been difficult to mass-produce the optical pickup.